Video teleconferencing systems allow for simultaneous exchange of audio, video and data information among multiple audiovisual terminals. Systems known as multipoint control units (MCUs) perform switching functions to allow three or more audiovisual terminals to intercommunicate in a conference. The central function of an MCU is to link multiple video teleconferencing sites together by receiving frames of digital signals from audiovisual terminals, processing the received signals, and retransmitting the processed signals to appropriate audiovisual terminals as frames of digital signals. The digital signals can include audio, video, data and control information. Video signals from two or more audiovisual terminals can be spatially mixed to form a composite video signal for viewing by teleconference participants.
Advances in digital communications have led to the proliferation of digital audiovisual terminals with codecs employing data compression. The Telecommunications Standardization Sector (TSS) of the International Telecommunication Union has specified a series of recommendations for video teleconferencing known as the H-Series. The H-Series includes H.221 defining frame structure, H.261 defining video coding and decoding, H.231 defining multipoint control units, and H.320 defining audiovisual terminals. Standards-based compression algorithms (e.g., H.261) are becoming widespread. However, there are many proprietary algorithms for which better quality or compression rates are claimed. It is, therefore, desirable to connect terminals having incompatible compression algorithms. The typical MCU can support multiple conferences in which separate conferences can have different video compression algorithms, audio encoding algorithms, transmission rates, and protocols. Unfortunately, because the hardware characteristics of the audiovisual terminals are typically different from one another (transmission rate, compression algorithm, protocol or resolution), it has not usually been possible to interconnect different audiovisual terminals in a single conference. Because of these limitations, subscribers have been faced with the costly task of installing multiple types of equipment associated with different compression algorithms or transmission rates.
Network based services offered by interexchange carriers exist that allow transcoding between different compression algorithms of audiovisual terminals in a conference. These known transcoding services operate by first decoding compressed signals from each audiovisual terminal according to its respective compression algorithm and then converting the resultant uncompressed signals into analog signals. For example, the analog signal produced from a terminal A having coding algorithm X may be encoded by an algorithm Y associated with terminal B, thus achieving transcoding between unlike terminals A and B. Such an analog transcoding scheme can also be used for matching transmission rates between different codecs.